Description: A disease defined as a head circumference more than 3 standard deviations below the age-related mean. Brain weight is markedly reduced and the cerebral cortex is disproportionately small. Despite this marked reduction in size, the gyral pattern is relatively well preserved, with no major abnormality in cortical architecture. Affected individuals are mentally retarded. Primary microcephaly is further defined by the absence of other syndromic features or significant neurological deficits due to degenerative brain disorder. Some MCHP1 patients also present growth retardation, short stature, and misregulated chromosome condensation as indicated by a high number of prophase-like cells detected in routine cytogenetic preparations and poor-quality metaphase G-banding. {ECO:0000269|PubMed:12046007, ECO:0000269|PubMed:15199523, ECO:0000269|PubMed:16211557}. Note=The disease is caused by mutations affecting the gene represented in this entry.